Made _discard_pending() always post a message. Added meaningful
default messages to all the places where it is called in the code. In
particular, CQ will now always post a message whenever the Commit
checkbox is unchecked, for any reason.
Updated unit tests accordingly, and made them a bit more robust by
adding an error message to the failing FakeVerifier.
This does NOT fix the bug, but will hopefully give more visibility to
developers when a CL is dropped.